<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>画个三角形</title>
    <style>
        .sanjiao{
            height: 0;
            width: 0;
            border-left: 20px solid red;
            border-top: 10px solid transparent;
            border-bottom: 10px solid transparent;
        }
    </style>
</head>
<body>
    <div class='sanjiao'></div>
</body>
</html>